Radio-frequency identification (RFID) systems commonly use tags, or labels attached to the objects for identification of those objects. For such systems, two-way radio transceivers, also referred to as interrogators or readers, send a signal to the tag and read its response.
RFID tags can be passive, active or battery-assisted passive. An active tag has an on-board battery and periodically transmits its ID signal. A battery-assisted passive RFID tag can have a small battery and is activated when in the presence of an RFID reader. A passive tag is typically cheaper and smaller because it has no battery; instead, the tag uses the radio energy transmitted by the reader. However, to operate a passive tag, it must be illuminated with a power level roughly a thousand times stronger than for signal transmission. That makes a difference in interference and in exposure to radiation.
Tags may either be read-only, having a factory-assigned serial number that is used as a key into a database, or may be read/write, where object-specific data can be written into the tag by the system user. Field programmable tags may be write-once, read-multiple; “blank” tags may be written with an electronic product code by the user.
RFID tags typically contain at least two parts: an integrated circuit for storing and processing information, modulating and demodulating a radio-frequency (RF) signal, collecting DC power from the incident reader signal, and other specialized functions; and an antenna for receiving and transmitting the signal. The tag information is stored in a non-volatile memory. The RFID tag includes either fixed or programmable logic for processing the transmission and sensor data, respectively.
An RFID reader transmits an encoded radio signal to interrogate the tag. The RFID tag receives the message and then responds with its identification and other information. This may be only a unique tag serial number, or may be product-related information such as a stock number, lot or batch number, production date, or other specific information. Since tags have individual serial numbers, the RFID system design can discriminate among several tags that might be within the range of the RFID reader and read them simultaneously.
RFID systems can be classified by the type of tag and reader. A passive-reader active-tag system has a passive reader which only receives radio signals from active tags (e.g., battery operated, transmit only). The reception range of a passive-reader active-tag system reader can be adjusted, e.g., from 1-2,000 feet (0-600 m), allowing flexibility in applications such as asset protection and supervision. An active-reader passive-tag system has an active reader, which transmits interrogator signals and also receives authentication replies from passive tags. An active-reader active-tag system employs active tags which are awaken with an interrogation signal from an active reader. A variation of this system could also use a battery-assisted passive tag which acts like a passive tag but has a small battery to power the tag's return reporting signal.
Fixed readers are typically set up to create a specific interrogation zone which can be tightly controlled. This allows a highly defined reading area for when tags go in and out of the interrogation zone. Mobile readers may be hand-held or mounted on carts or vehicles.
Active tags may contain functionally separated transmitters and receivers, and the tag need not respond on a frequency related to the reader's interrogation signal. An Electronic Product Code (EPC) is one common type of data that may be stored in a tag.
Often more than one tag will respond to a tag reader, for example, many individual products with tags may be shipped in a common box or on a common pallet. Collision detection is important to allow reading of data. Two different types of protocols are often used to “singulate” a particular tag (i.e., differentiate it from other nearby tags), allowing its data to be read in the midst of many similar tags. In a slotted-ALOHA system, the reader broadcasts an initialization command and a parameter that the tags individually use to pseudo-randomly delay their responses. When using an “adaptive binary tree” protocol, the reader sends an initialization symbol and then transmits one bit of ID data at a time; only tags with matching bits respond, and eventually only one tag matches the complete ID string.
Prior RFID systems have presented drawbacks when used with many tags or with multiple overlapping readers. Additionally, prior RFID systems have encountered difficulties when an RFID reader has attempted to read one or more RFID tags contained within an enclosure or structure. Problems may in particular arise if the enclosure or structure is made with metallic structure(s) or elements, which can scatter or reflect incident RF energy from the reader or prevent reception by the RFID tags due to the structure(s) or elements acting as a Faraday cage.